The removal of particulate matter from air streams is typically done now with baghouses or electrostatic precipitators and from liquids with screens or filters. Baghouses usually employ fabrics which serve as the support for the buildup of a filter cake. Periodically, the filter cake is shaken or blown loose from the fabric and collected, and the cycle is repeated.
Typical pressure drops are on the order of 10-20 cm WC (water column). As might be expected, there is some loss of efficiency after the cake is removed during the cleaning cycle and there tends to be a relatively high pressure drop during the latter phase of filtration just prior to cleaning. Furthermore, baghouse fabrics may blind due to lodging of fine particles or sticky materials in the pores of the fabric.
In addition to commercial baghouses and electrostatic precipitators, research and development work has been done on recirculating, granular-bed filters. These filters comprise a bed of granules in which the granules are recycled out the bottom of the bed, cleaned, and returned to the top. Dirty gas flows from side to side or from bottom to top, countercurrent to the granule movement.